gammaincinv

Syntax

x = gammaincinv(y,a)
x = gammaincinv(y,a,tail)

Description

x = gammaincinv(y,a) evaluates the inverse
incomplete gamma function for corresponding elements of y and a,
such that y = gammainc(x,a). The elements of y must
be in the closed interval [0,1], and those of a must
be nonnegative. y and a must
be real and the same size (or either can be a scalar).

x = gammaincinv(y,a,tail) specifies the
tail of the incomplete gamma function. Choices are 'lower' (the
default) to use the integral from 0 to x, or 'upper' to
use the integral from x to infinity.

These two choices are related as:

gammaincinv(y,a,'upper') = gammaincinv(1-y,a,'lower').

When y is close to 0, the 'upper' option
provides a way to compute x more accurately than
by subtracting y from 1.